In a typical radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink and/or uplink cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
During the last few years cellular operators have started to offer mobile broadband based on WCDMA/HSPA. Further, fuelled by new devices designed for data applications, the end user performance requirements are steadily increasing. The large uptake of mobile broadband has resulted in heavy traffic volumes that need to be handled by the HSPA networks have grown significantly. Therefore, techniques that allow cellular operators to manage their spectrum resources more efficiency are of large importance.
Few such techniques whereby it is possible to improve the downlink performance would be to introduce support for 4-branch Multiple Input Multiple Output (MIMO), multi-flow communication, multi carrier deployment etc. Since improvements in spectral efficiency per link are approaching theoretical limits, the next generation technology is about improving the spectral efficiency per unit area. In other words, the additional features for High Speed Downlink Packet Access (HSDPA) need to provide a uniform user experience to users anywhere inside a cell by changing the topology of traditional networks. Currently 3GPP has been working on this aspect using Heterogeneous networks (HetNet) [1]-[3].
A homogeneous network is a network of base stations or network nodes (Node B) in a planned layout and a collection of user terminals e.g. UEs in which network all base stations have similar transmit power levels, antenna patterns, receiver noise floors, and similar backhaul connectivity to the data network. Note that the data network can be either core network either via RNC or directly to the core network. Moreover, all base stations offer unrestricted access to user terminals in the network, and serve roughly the same number of user terminals or UEs. Current wireless systems that belong to this category are for example as mentioned above, GSM, WCDMA, HASDPA, LTE, Wimax. Etc.
Another type of networks are known as heterogeneous networks: In Heterogeneous networks, in addition to the planned or regular placement of macro base stations mentioned above relating to homogeneous networks, several micro/pico/femto/relay/small cell base stations are deployed. This is depicted in FIG. 1.
The power transmitted by these micro/pico/femto/relay/small cell base stations is relatively small which may be up to 2 Watts, compared to that of macro base stations which may be up to 40 Watts. Due to the difference in transmit power, these micro/pico/femto/relay/small cell base stations may be viewed as Low Power Nodes (LPNs). An advantage of using LPNS is that when they are deployed in the network they are designed to or configured to eliminate coverage holes in the homogeneous networks (using macro only) as well as to improve the capacity in traffic hot-spots. Hence they complement the macro base stations to improve the capacity in the heterogeneous network. Also, due to their lower transmit power and smaller physical size, these LPNs offer flexible site acquisitions.
A LPN in a heterogeneous network may have a different cell identifier as that of macro base station which makes the cell formed by the LPN and the macro base station respectively different.
The LPN in such a network may also have the same cell identifier as that of a macro base station and in such case the cell formed by the LPN and the macro network node is soft cell or shared cell or combined cell.
In other words, in a combined cell of a heterogeneous network, a LPN may have a different cell ID and a different scrambling code as that of a macro network node. This is a first category of a heterogeneous network. Another category is that all LPNs share the same cell ID as that of the macro network node. These different categories are depicted in FIG. 2 and FIG. 3 respectively.
As shown FIG. 2 shows a heterogeneous network where LPNs create different cells B and C, whereas the macro network node creates a cell A having a different ID than that of B and/or C. Simulations show that deploying LPNs in a macro cell offers load balancing, hence achieving huge gains in system throughout as well as cell edge user throughput.
A drawback with the scenario of FIG. 2 is that since each cell LPN creates a different cell, higher layer signaling is needed to perform handover of a UE when moving from one LPN to macro node or to another LPN.
In FIG. 3 the heterogeneous network comprises LPNs that are part of the macro cell A since the same cell ID is used for all cells. This deployment is sometimes referred to as a soft cell, shared cell, or combined-cell deployment. This deployment may be seen as a distributed Multi Input Multi Output (MIMO). This scenario may be used for different applications. For example two transmit antennas can be set up at Macro network node, while another two antennas may be installed at LPN. In this way a distributed MIMO system is created. Moreover this scenario avoids the frequent soft handovers, hence also avoids higher layer signaling.
Referring to FIG. 4 there is depicted a configuration or scenario of a combined cell deployment where a central controller in the combined cell is configured to collect operational statistics information of network environment measurements from the other network nodes (here shown surrounding the central controller). The central control may be a network node connected to a LPN or it may be a network node connected to a combination of LPNs and macro nodes. The central controller may also be a RNC or any suitable network node. The decision of which network node(s) to serve and transmit data and/or control signalling to a specific UE may therefore be made by the central controller based on the information provided by the UE or on its own. The cooperation among various network nodes is instructed by the central controller and implemented in a centralized way.
In a combined cell deployment, transmitting the same signal from each network node causes wastage of resources and fails to provide capacity benefits when the load of the cell is high. One approach to increase the capacity of the combined cell deployment is to reuse the resources e.g. spreading or channelization codes among various network nodes. This is sometimes called spatial reuse.
FIG. 5 shows the configuration of spatial reuse between two network nodes A and B in a combined cell. These two network nodes share the same scrambling codes and also spreading or channelization codes. This is denoted in FIG. 5 as Si, Ci, where i=1.
For efficient implementation of spatial reuse, the central scheduler or central node needs to schedule the users (or UEs) to the proper (respective) network nodes. For example if a user is located nearer to a network node and the data is scheduled from another network node to this user, such a scheduling decision would result in waste of resources as well as unnecessary interference to the other cells.
One method to identify which network node(s) is appropriate for transmitting to a particular UE is by transmitting probing pilots. Unfortunately only UEs which can understand these pilots i.e. the UEs which are released after these pilots are standardized in third Generation Partnership Project (3GPP), Release 12; also called Rel-12 UEs; might be scheduled using probing pilots. The UEs which are already in the market; also called legacy UEs; may not benefit from these pilots. Hence when these UEs are present in the network or system, the central node is today unable to use spatial reuse scheme to improve the system capacity.